Looking at this thing, it appears to be a programmable version of DropBox (https://www.dropbox.com/), a "cloud" repository that could be useful for people who did not have their own online storage, but
Explain concepts important to validation of email addresses with regular expressions. Applies to most languages/tools that uses regular expressions.
Consider email address RFCs:
Look at HTML5 form input element (with type=email) regex pattern: